1. Field of the Invention
This invention relates to solar panels. In particular, the invention relates to pumpless recirculation of heat transfer fluid from a heat exchange region, where it is condensed from a vapor to a liquid, back to a higher elevated solar collection region where the liquid is vaporized by solar radiation.
2. Prior Art
In the face of dwindling fossil fuels, a natural worldwide effort has evolved to harness the energy of the sun. Accordingly, a wide variety of so-called solar panels have been developed, many of them to a commercial extent. While the use to which they are made varies from generation of electricity, space heating, etc., their largest area of use lies in the solar heating of water.
Solar hot water heaters are either of the direct heating type wherein water circulates through the solar energy collection region; or of the indirect heating type where water acquires heat by heat exchange with a heat transfer liquid that circulates through the solar collection region. A particular variety of indirect solar heating is that based on the principle of a reflux condenser. That is, a fluid having a low boiling temperature is vaporized in a solar collector and passed through a heat exchanger (hereinafter referred to as a condenser) where it gives up its latent heat of vaporization to water by condensation. Reflux condenser solar water heaters are described, for example, in U.S. Pat. No. 4,080,957 (issued Mar. 28, 1978 to C. J. Bennett).
That a particular solar water heater is capable of its water heating task is no guarantee of commercial success nor is it assurance that overall price and energy consumption will be sufficiently attractive to the average user. In the reflux condenser solar water heater, configuration, bulk and the means by which heat transfer liquid condensate is recirculated to the solar collector are major concerns. In the C. J. Bennett solar heater, heat transfer liquid condensing within a jacket surrounding a water pipe passes back to the solar collection panel by gravity. Of necessity, therefore, the condenser is elevated higher than the solar collection region. Although this avoids the need for separately powered pumps to recirculate condensate, the advantage is outweighed by excessive bulk attributable to the positioning of condenser relative to collector. To reduce bulk, one may consider hypothetically to position the condenser at a lower elevation than the collector. Under that arrangement the condensate must be pumped or otherwise transported to the solar collector, gravity flow no longer being possible. Use of an electric pump, of course, is a common expedient but poses an additional installation step and requires electricity that detracts from the unit's efficiency. Vacation homeowners that have no electricity, moreover, cannot use such units.
A pumpless system for moving liquid, particularly water, as vapor upward from a lower elevation is described in U.S. Pat. No. 3,490,996 (issued Jan. 20, 1970 to H. C. Kelly, Jr.). The patent discloses collecting water vapor from a reservoir and passing the vapor into a duct that concentrates solar energy and discharges the vapor into a higher elevated second reservoir. The concentration of solar energy on the duct continuously drives the water vapor upward at high velocity where it condenses in the second reservoir. An arrangement within the duct provides for entrainment of slower moving vapor in the high velocity vapor. It is clear from the teachings of the U.S. Pat. No. 3,490,996 patent that solar energy is primarily intended to facilitate vapor transport as opposed to liquid transport. If this principle were applied to a reflux condenser solar water heater, condensate from the condenser might flow to a lower portion of a solar collector, vaporize, and pass through the major upper portion of the collector as vapor before returning to the condenser. Advantage of the large surface area of the collector to vaporize large quantities of heat transfer liquid per unit time may thus be lost, producing what would appear to be a less efficient solar collector compared to one in which heat transfer fluid is introduced as liquid at the top of the collector and distributed by gravity throughout substantially all of the collection region before vaporizing.